You will need a foam coffee or milk shake container, some string, two bendy drinking straws and a pencil.
Near the bottom of the cup poke two holes with your pencil just a little smaller than the straws.
Snip the long ends of the straws so that the straws on either side of the bendy bits are the same length and insert them into the holes, as left.
Above these holes punch two more just under the beaded lip and tie a piece of string loosely across. To the centre of this 'handle' tie a longer piece, say a metre or so.
Unless you have very understanding adults take your Hero's Engine outside and suspend it by the long bit of string to something like a tree or washing line.
Fill your container with water and watch it rotate as it empties.
You are watching gravity attempting to drag your engine to the ground.
As the engine is secured and won't move, gravity settles for taking all the water and is pulling it down.
As the water spurts out through your straws in one direction Newton's third law is working in the opposite direction.